Exemplary embodiments relate to memory, and more specifically, to bidirectional writing for a magnetic memory device.
Magnetic Random Access Memory (MRAM) is a non-volatile computer memory (NVRAM) technology. Unlike conventional RAM chip technologies, in MRAM data is not stored as electric charge or current flows, but by magnetic storage elements. The elements are formed using two ferromagnetic plates, separated by a thin insulating layer. One of the two plates is a permanent magnet set to a particular polarity; the other's field can be changed to match that of an external field to store digital data. This configuration is known as a spin valve and is the simplest structure for a MRAM bit. A memory device is built from a grid of such “cells”.
The simplest method of reading is accomplished by measuring the electrical resistance of the cell. A particular cell is (typically) selected by turning on an associated access transistor which passes current from a supply line through the cell to ground. Due to the magnetic tunnel effect, the electrical resistance of the cell changes with the relative orientation of the fields between the two plates. By measuring the resulting current, the resistance associated with any particular cell can be determined, and from this magnetic orientation of the writable plate. Typically if the two plates have the same orientation the cell is considered to hold a value of “0”, while if the two plates are of opposite orientation the resistance will be higher and will hold a value of “1”.
Data is written to the cells using a variety of means. In one method for field-written MRAM, each cell lies between a pair of write lines arranged at right angles to each other, above and below the cell. When current is passed through them, an induced magnetic field is created at the junction, which acts on the writable plate and under proper conditions will cause the writable plate to reverse its orientation.